fix AsyncSeq.scanAsync #11

Author: eulerfx

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -306,14 +306,17 @@ module AsyncSeq =
   ///
   /// The aggregation function is asynchronous (and the input sequence will
   /// be asked for the next element after the processing of an element completes).
-  let rec scanAsync f (state:'TState) (input : AsyncSeq<'T>) = asyncSeq {
-    let! v = input
-    match v with
-    | Nil -> ()
-    | Cons(h, t) ->
+  let scanAsync f (state:'TState) (input : AsyncSeq<'T>) =    
+    let rec go f state s = asyncSeq {
+      let! v = s
+      match v with
+      | Nil -> ()
+      | Cons(h, t) ->
         let! v = f state h
         yield v
-        yield! t |> scanAsync f v }
+        yield! t |> go f v }
+    Cons(state, go f state input) |> async.Return
+
 
   /// Iterates over the input sequence and calls the specified function for
   /// every value (to perform some side-effect asynchronously).

tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs

@@ -153,7 +153,16 @@ let ``AsyncSeq.scanAsync``() =
   let f i a = i + a
   let z = 0
   let actual = ls |> AsyncSeq.ofSeq |> AsyncSeq.scanAsync (fun i a -> f i a |> async.Return) z
-  let expected = ls |> Seq.scan f z |> Seq.skip 1 |> AsyncSeq.ofSeq
+  let expected = ls |> List.scan f z |> AsyncSeq.ofSeq
+  Assert.True(EQ expected actual)
+
+[<Test>]
+let ``AsyncSeq.scanAsync on empty should emit initial state``() =
+  let ls = List.empty
+  let f i a = i + a
+  let z = 0
+  let actual = ls |> AsyncSeq.ofSeq |> AsyncSeq.scanAsync (fun i a -> f i a |> async.Return) z
+  let expected = ls |> List.scan f z |> AsyncSeq.ofSeq
   Assert.True(EQ expected actual)